Roots-type superchargers are used to pump air into an engine at a rate greater than natural aspiration. The air enters the supercharger at nearly atmospheric pressure. Rotors in the supercharger carry nearly atmospheric air to an outlet port where the air is pressurized for delivery to the cylinders of an engine. The discharge of the nearly atmospheric air into the pressurized outlet creates back flow noise in the form of a pneumatic report or pop. When repeated at the high frequency which is typical of supercharger operation, this series of reports becomes a whine.
Fixed back flow ports have been used to reduce noise by allowing a small amount of pressurized air to flow from the outlet of the supercharger into the nearly atmospheric rotor chambers. This tends to equalize the pressure between the outlet and the rotor chambers gradually so that when the rotor chambers exhaust, there is not as much energy in each pulse, thereby reducing noise. However, fixed ports can be optimized for only a small range of engine speeds. A small fixed port close to the outlet port can reduce noise at low engine speeds but, at a high engine speeds, small retarded ports are inadequate to equalize pressure between the outlet and the cavity, so that increased noise levels result. However, a large and advanced fixed port, used to quiet the supercharger at high speeds, will result in excessive back flow and reduced efficiency at lower engine speeds.